Strangely, considering how much of an impact this heist makes on the game, and how difficult it might prove, you can get away with hiring completely inexperienced crew members without much consequence. It can bite you if you only hire the cheapest hands, but you still can do so while potentially making off with 100% of the take.
Subtle:
This is the sneaky, easy way take on the Union Depository. You'll need to hire 2 guns, 2 drivers, and 1 hacker, so your funds will be stretched thin depending on who you choose. The "first" gun and driver make the most difference, so they're the ones you need to be careful in your selection. You can, however, hire anyone, including the discount crew members, for the "second" gun and driver.
The "second" Gunman and Driver will replace the blown tires of the Security Vans you take over and drive off with the hostages, so their roles will be taken care of almost immediately, and they basically can't fail you. You'll have zero problems taking the gold, since as Lester describes you literally trick them into "giving it to you", but you'll run into trouble when you try to offload the gold into the getaway cars. You'll need to assume control of the street lights to block the pursuing Merryweather Security while expediating your own escape, but once more your Hacker will only cause potential delays, at worst. If you've picked Ricky for the 2 opportunities you could up until this point, he will handle his responsibilities without any hang-up, so that's not a problem.
Once you reach the getaway cars, things get a little bit complicated. The 3 main characters need to hold off several heavily-armed waves of Merryweather while your "first" Gunman and Driver offload the gold from the vans into the cars. The area has ample cover, so this shouldn't prove difficult to hold off at all. If your crew here is inexperienced, they won't be able to transfer more than half of the gold before you need to get in the cars are drive off, so you need good ones to finish by the time you're ready if you're to make off with the maximum $201,600,000 in gold. You should absolutely have Taliana unlocked at this point, so she will handle herself just fine. You can hire either Packie or POTENTIALLY the low-end Gun IF he took part in the parachuting Bureau Raid approach and upgraded his abilities, and you should transfer 100% of the gold to the getaway cars.
The getaway is a simple matter of following the other 3 cars as Franklin, and if you hired Taliana for your "first" Driver, all 4 of you will make your escape without any issue. All told, you can save enough to make at least $34million on each character, at the end of this heist.
Obvious:
This is the bold approach, and it's more difficult, but with the higher risk comes higher reward. You won't need a Hacker with this approach, so you already save on requiring 1 fewer crew member on this heist. Despite being the biggest and most daring heist in the game, in addition to saving on crew size, you don't need to spend on crew quality either! NONE of your crew members need to be very expensive, so as long as you haven't lost any cheap Gunmen in earlier heists, and you've unlocked Taliana, you should make your getaway with the full $201,600,000 in gold, and each character will make $40million in the end.
During the various phases of the heist, your hired guns will be looked out for, and hiring cheap and/or inexperienced hands won't cost you in any way. Franklin will be covering one of the guns you hired while he loads the gold into cages for helicopter pickup, then he'll split once Trevor and your "first" driver (pilot) collect the gold. The other gun will follow Michael and Franklin as they make their getaway from their distraction maneuver while the gold flies away.
Your "first" driver makes the biggest difference, since he/she will be piloting a helicopter carrying half of the take, so if you hire an inexperienced "driver", they'll crash and you'll lose half of the take. This shouldn't really be a problem at all, however, because Taliana is both the cheapest driver and has the best default driving skill of any driver. She won't crash unless you don't look out for her as Trevor. The "second" driver is just filler, as he will be driving a train, so hire the cheapest guy you can, and you're all set to go. Once Michael's crew makes their extended getaway and loses police pursuit and once Trevor gets rid of the tailing Merryweather helicopters and drops off the gold, you're done. You should be able to complete this heist with roughly $40million in the bank for each of your characters.
Summary:
Take the Obvious approach, hire Taliana for Driver #1, and fill the rest of the slots with the cheapest hands you have available. You'll make a killing in saving on crew numbers and expertise, and it won't cost you unlike earlier heists.
